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Down the halogen group , electropositive character increases , then why is it that Fluoride ion is the least stable ? The counter arguement is that fluoride ion has four lone pairs and a small size which makes it least stable.Down the group , since atomic size increases the extra electron is stabilized. In all such stability trends there are multiple counteracting factors , how to decide which one to give more weightage ?

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Actually, the answer is a bit more nuanced. If we observe the pKb values (i.e. tendency to form HX from X-, where X is a B-L base) it is pretty clear that F- would form HF very quickly.

The explanation is that I- would have a much larger area to balance out the negative charge compared to F-, and the anion would be more stable as a result.

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